1. Field of the Invention
The present invention relates to a method for automatically conducting single particle experiments which includes subjecting the particles to external forces, using laser Doppler velocimetery to monitor the motion of the particles, summarizing the particle response to a database, analyzing the information contained in the database and, thus, determining physical properties of the particles in a sample. These physical properties may include particle size, density, charge, magnetic moment and various optical scattering attributes.
2. Description of the Related Art
This and other forced motion particle measurement schemes are descendants of R. A. Milikan's oil drop experiment of 1911. The original experiment exposed small particles simultaneously to gravitational, viscous and electrical forces. By direct observation, the settling velocity and the electrical migration velocity of single particles was measured. Data were interpreted using Stokes law and Coulombs law to yield the aerodynamic diameter and the electric mobility of the individual particles. Many extensions and improvements of the Milikan cell have been used in both experimental physics and in analytical instruments for fine particle assay.
Forced motion experiments other than those involving electricity and gravity make use of acoustic force, magnetic force and fluid gradient forces. Instruments such as the TSI Aerodynamic Particle Sizer (available from Thermo Systems Inc., Minneapolis, Minn.), the Malvern Aerosizer (available from the Malvern Instruments Ltd., Malvern, U.K. are in this category.
Historically, the most important document on the acoustical aerosol sizing technique is Gucker and Doyle, "Amplitude of Vibration of Aerosol Droplets in a Sonic Field", Journal of Physical Chemistry, vol. 60. pp 989-996, (1956). In conclusion, the authors describe an instrument which would use the combination of a linear gradient filter and a photomultiplier tube to accomplish the transduction of particle velocity into an electrical signal. Further description suggests that the velocity magnitude may be processed by "a rapid electronic circuit to obtain the combined count and size distribution, while the phase lag of the particle also could be studied with a rapid electronic phase meter".
The use of LDV (laser Doppler velocimitry) to implement the acoustical sizing technique was first described by Kirsch and Mazumder, "Aerosol Size Spectrum Analysis Using Relaxation Time Measurement", Applied Physics Letters, Vol. 26, No. 4, 15 Feb. 1975. U.S. Pat. No. 4,663,741 discloses an extension of this method for performing simultaneous charge and size measurements in which a combination of an acoustic field and a static electrical field are used.
Sasaki and Sato in "Laser Doppler Particle Measurement Using Forced Vibrations and Power Spectral Analysis", Appl. Optics, Vol. 17, no. 2, pages 230-234 (Jan. 15, 1978), and in "Simultaneous Determination of Particle Size and Density by LDV", Appl. Optics, Vol. 19, no. 15, pages 2565-2568 (Aug. 1, 1980) describe some improvements to the LDV assisted acoustic motion technique. Wilson and Liu in "Aerodynamic Particle Size Measurement by Laser-Doppler Velocimetry", Journal of Aerosol Science, Vol. 11, pp.139-150 (1980), review other previous work in this field pertaining to LDV assisted forced motion. In all cases the LDV system is used to replace the eye or camera used in early work. The experimental formats were formulated before the advent of lasers and hence LDV. Examples of prototype single particle instruments are referenced in N. A. Fuchs, Mechanics of Aerosols, Dover (1964). Experiments involving combinations of alternating electrical fields, magnetic fields, gravity, thermal gradients and acoustic forces are all described and referenced therein.
So far this discussion has omitted an important class of optical particle sizing instruments: Those which use the magnitude and spatial distribution of scattered light to estimate particle size. An excellent summary of the history of this class is to be found in Milton Kerker, The Scattering of Light, Academic Press (1969). With regards to the current invention, the reader is directed to Son and Giel, "Circuit for Simultaneous Measurements of Particle Sizing Interferometer Signal Characteristics", Review of Scientific Instruments, Vol. 58 Number 3, pages 393-400 (1987) and Bachalo's U.S. Pat. No. 4,854,704, for a review of similar art.